Alex Sanchez, known nationally for his anti-gang work with Homies Unidos, is held on conspiracy and racketeering charges. He allegedly was involved with the Mara Salvatrucha gang.

A nationally known anti-gang activist was arrested Wednesday on federal racketeering and conspiracy charges stemming from his alleged involvement in one of the most violent street gangs in the United States.

Alex Sanchez, executive director of Homies Unidos, a gang-intervention nonprofit with offices in Los Angeles and El Salvador, was among two dozen alleged members or associates of the Mara Salvatrucha gang, also known as MS-13, charged in a 66-page indictment that was unsealed Wednesday.

The defendants, with monikers such as Creeper, Grinch, Pain and Tears, were involved in a variety of crimes, including murder, conspiracy to commit murder, extortion and drug trafficking, over a 15-year period, the indictment alleges. Among the alleged crimes was a plot to kill a Los Angeles Police Department detective who specialized in investigating the gang, authorities said. Gang members had gone so far as choosing a handgun with which to kill Det. Frank Flores, authorities allege, but police thwarted the plot.

Sanchez's arrest comes as officials seek to train former gang members to work with at-risk youths. He is at least the fourth anti-gang worker to be charged with gang-related crimes in the last several years.

Identified in court papers by the gang name Rebelde, or rebel, Sanchez was allegedly a gang leader, known as a "shot caller." He is accused of conspiring with fellow gang members to kill a man in El Salvador in 2006 and other crimes.

Daniel McMullen, a top FBI official in Los Angeles, called Sanchez a gang member "who used the guise of a reformist to deceive the public and to play both sides by accepting funds from well-intentioned donors . . . while allegedly committing crimes on behalf of the gang."

Sanchez, 37, was arrested at his home in Bellflower. He broke into tears at a U.S. District Court hearing at which he was read the charges. He is expected to enter a plea next week. The board of directors of Homies Unidos issued a statement Wednesday evening expressing "full support" for Sanchez and saying that its members were "confident in Alex's innocence."

Homies Unidos received about $1.6 million in tax-deductible donations from 2002 through 2007, according to tax returns available on Guidestar, an online database of nonprofits. Some of the money came from celebrities, according to law enforcement sources who asked not to be named because they were not authorized to speak about the matter. It was not immediately clear how much money came from government sources, but at least $10,000 came from Los Angeles, according to the city controller's office.

Before his arrest, Sanchez had been winning accolades from politicians and others for years for his work with Homies Unidos. Based in an office near MacArthur Park, the organization performed tattoo removal for former gangsters who wanted a fresh start and offered life-skills courses for those leaving the gang life.

"For 10 years Homies Unidos has been a catalyst for change, working to end violence and promote peace in our communities," Sanchez wrote on the group's website. "In fact we are living proof that prevention works and that peace is possible."

Sanchez, a former Mara Salvatrucha gang member in the early 1990s, first came to public attention during the LAPD Rampart corruption scandal a decade ago. Allegedly reformed and working at Homies Unidos, he was planning to testify on behalf of a teenager who contended that he had been falsely implicated in a killing.

But before Sanchez could do so, he was arrested by the LAPD and turned over to immigration authorities, who began deportation proceedings for the illegal immigrant who had reentered the United States after having been deported following a car-theft conviction.

Sanchez applied for political asylum, arguing that he might be killed if he was returned to El Salvador because of his former involvement with the Mara Salvatrucha and his outspoken stance against police corruption.

His case became something of a cause celebre when police critics cited it as an example of how officers in the Rampart Division tried to neutralize potentially problematic witnesses by having them deported.

Among his chief supporters at the time was then-state Sen. Tom Hayden, who accused LAPD officers of improperly targeting Sanchez for arrest. Hayden supported Sanchez's bid for asylum, which was approved in 2002.

Hayden said Wednesday that it was too soon to assess the merit of the case against Sanchez, but that he does not believe the allegations negated his past achievements.

"He's one of the most sensitive, knowledgeable and effective anti-violence practitioners I've met," Hayden said in a telephone interview. "The violence he has averted in Los Angeles and El Salvador is hard to quantify, but there's no doubt he's done it."

L.A. City Councilman Ed Reyes, who said he gave $5,000 in public funds to Homies Unidos for a children's soccer program two years ago, said he was "totally shocked" by the charges.